Low resistivity potentiometer wire



March 7, 1961 B. BRENNER 2,974,298

LOW RESISTIVITY POTENTIOMETER WIRE Filed Jan. 30, 1959 2 Sheets-Sheet 1 l """"I. m "V -0/14 Be rt Bren ner INVENTOR.

ATTORNEY March 7, 1961 B. BRENNER 2,974,298

LOW RESISTIVITY POTENTIOMETER WIRE Filed Jan. 30, 1959 I l I I o o o a) a:

00021 JLUQ/SLULIO 2 Sheets-Sheet 2 o 1 0 WEIGHT PERCENT COPPER IN VENTOR.

*LIAUSISBH Bert Brenner ATTORNEY United States Patent LOW RESISTIVITY POTENTIOMETER WIRE Bert Brenner, Tenafly, N.J., assignor to Sigmund Calm Corp., Mount Vernon, N.Y., a corporation of New York Filed Jan. 30, 1959, Ser. No. 790,149

Claims. (Cl. 338-160) This invention relates to the art of variable resistance devices, such as rheostats and potentiometers. The invention pertains in one of its specific aspects to novel winding or resistance elements for potentiometers having a low overall resistance value and used, for example, in telemetering and miniaturized control systems.

A potentiometer includes a variable resistance element in which the resistance value is varied by a sliding contact or wiper which is movable over the resistance element. The resistance element may take the form of a wire which is wound or otherwise supported on a dielectric support. The resistance element may be of any other type known to the art, such as the metal film type. Accordingly, the term wire, appearing in this description and in the claims, embraces various forms or types of resistance elements. The physical shape of the resistance element may be relatively elongated on a linear support or toroidal on an annular support of metal or dielectric material, such as a suitable plastic.

Among the desirable properties and characteristics of a potentiometer, particularly of the low torque, low over all resistance type used in telemetering and miniaturized control systems, are low contact resistance, good electrical linearity, wear resistance, and low noise. In wire wound potentiometers intended to meet the foregoing requirements, the wire should have excellent corrosion resistance, low contact resistance with respect to the wiper, good linearity, high tensile strength so that it may be readily wound in small size without danger of breaking or stretching, and the ability to withstand wear.

The enumerated properties and characteristics are difficult to attain in a potentiometer wire of low resistivity, such as a resistivity of the order of 80 ohms or less per circular mil foot. In particular, known alloys having low unit resistance, such as copper alloyed with a small percentage of nickel, are usually too soft for adequate wear resistance, have low tensile strength, and have very poor corrosion resistance. Tensile strength is of great importance in order that a small diameter wire may be drawn and wound on a support form without undue breakage. For example, the required wire diameter may be of the order of 0.00035 to 0.010" and, without adequate inherent tensile strength, such fine wire would readily break during winding on a support, susceptibility to oxidation and corrosion (as exists with copper-nickel alloys) results in a high noise level, due to the formation of oxide films which are dielectric in nature and tend to cause poor contact between the wiper and the winding.

On the other hand, alloys having the requisite good wearing properties, high tensile strength, and good corrosion resistance have hitherto had relatively high rcsistivity, making them unsuitable for low overall resistance potentiometers.

In accordance with the present invention, it has been found that low resistivity, i.e. less than 80 ohms/c.m.f., can be obtained in ordered alloys of the copper-gold series. ,As is known to those skilled in the art, ordering tends to change many properties and characteristics of 'an alloy, such as its resistivity, tensile strength, elongation, hardness, specific heat, magnetic properties, and temperature coetficient of resistance. By an ordered alloy is meant one in which the atoms of the alloy constituents are arranged in orderly, periodic and systematic relation to each other in their crystal lattice positions rather than in random fashion. For example, in some ordered binary alloys, an atom of either constituent will have nearest to it only atoms of the other constituent and vice versa.

More particularly, in the present invention it has been found that certain copper-gold alloys, hereinafter referred to as CuAu alloys, attain ordering to a very marked degree when subjected to a definite heat treatment, this very marked ordering resulting in a decrease of over 50 percent in restivity when the alloy is transformed from the disordered state to the ordered state. Thus, these Cu--Au alloys, which have good corrosion resistance, high tensile strength, and long life when used as the resistance element of a potentiometer, are made suitable for low overall resistance potentiometers, by the marked decrease in their resistivity as a result of the ordering treatment. In addition the alloys have very low thermal versus copper.

To change these alloys from the disordered state to the ordered state, the heat treatment in accordance with the invention involves heating the alloys at about 700 degrees F. for at least one half hour followed by slow cooling for a period of about two hours. While this heat treatment reduces the resistivity of the alloys to the desired extent for the intended use of the alloy as the resistance element of a low overall resistance potentiometer, further heat treatment may reduce the resistivity somewhat further. However, it should be borne in mind that above 400 degrees C. (752 degrees F.) the alloys exist only in the disordered state.

The marked changes resulting from the ordering treatment are aptly illustrated by the following table of cerit is the primary object of this invention to provide an improved, low resistivity, alloy for use as a resistance element in a potentiometer.

Another object of the invention is to provide a low resitivity Wire of an ordered, binary, noble metal alloy for use in a potentiometer and having desired physical properties and electrical characteristics.

A further object of the invention is to provide a method of manufacturing a potentiometer winding wire of thecharacter indicated above.

For an understanding of the invention principles, reference is had to the following description of typical embodiments thereof as illustrated in the accompanying drawing, wherein:

Fig. 1 is a somewhat schematic plan view of one form of simple potentiometer employing a winding of an ordered alloy in accordance with the invention;

'Fig. 2 is a similar view of a simple toroidal potentiometer; and

Fig. 3 is a graph on which the resistivities in ohms/c.m.f.

of pluralities of disordered and ordered Cu-Au alloy as /gees resistance elements are plotted against the weight percent of copper in corresponding resistance elements.

Referring to Fig. 1, a linear potentiometer '10 is illustrated as comprising a support 11 on which is wound a fine diameter wire 12 that is hard drawn from a Au--Cu alloy within the percentage ranges of the invention and subjected to the aforementioned ordering treatment. The ends of wire 12 are connected to terminals 13 and 14. Wire 12 is insulated, as by enamelling, except where its closely spaced convolutions are contacted by a Wiper 15 extending from an operating handle 16 and movable parallel to support 11 along a guide rod 17. A wire 18 connects wiper 15 to a third terminal 19.

In Fig. 2, a toroidal potentiometer 20 is illustrated as including an annular support 21 wound with closely spaced convolutions of a wire 22 identical to wire 12. The ends of wire 22 are connected to terminals 23 and 24. A wiper 25 is rotatable by a knob 26 and contacts uninsulated portions of winding 22. A wire 28 connects wiper 25 to a third terminal 29.

Wipers 15 and 25 are low mass, electrically conductive elements of a suitable alloy having good wear and corrosion resistance properties, a long life, and a high polish. Preferably, the wipers 15 and 2.5 have a composition differing from that of wires 12 and 22.

Wires 12 and 22 are made from an alloy containing from 15% to 56% by weight of copper, the balance being essentially all gold, and are ordered in accordance with the procedure described above. Preferred ranges of constituents are from 1525% by weight of copper, the balance being essentially all gold, and from 50-56% by weight of copper, the balance being essentially all gold. Specific preferred compositions by weight are approximately 25% (Eu-75% Au and 50% Cu50% Au.

The merits of the above-mentioned preferred ranges of constituents and specific compositions will be recognized from a consideration of Fig. 3 which depicts two curves, namely curve I and curve 11. Curve I was obtained by plotting the resistivities at 20 C. in ohms/emf. of disordered copper-gold alloy compositions against the weight percent of copper in the compositions. Curve II was obtained by plotting the resistivities at 20 C. in ohms/emf. of ordered copper-gold alloy compositions against the Weight percent of copper in the compositions.

Curve I is also identified by points A, B, C and D. The portion of this curve between points B and C is derived from actual determinations. The broken line portions of this curve are extensions of the full line portion. Curve portion AB represents the probable resistivities of disordered alloys containing from copper, balance essentially all gold, to about 12%.% copper, balance essentially all gold. Curve portion C- D represents the probable resistivities of disordered alloys containing from about 56% copper, balance essentially all gold, to 100% copper and no gold.

Curve :II is generally W-shaped in configuration and indicates the resistivities and copper content of the ordered copper-gold alloys. This curve is also identified by points E, F, G, H and I. By ordering the alloys containing from to 56% copper and the balance essentially all gold, any desired resistivity between approximately 28 and 77 ohms/emf. may be obtained. The desired resistivity may be obtained by either of two methods-43y altering the composition of the fully ordered alloys or by ordering any selected composition only to an extent sufiicient to produce an alloy of requisite resistivity. Ordering has the effect of lowering the resistivity of the composition.

The first indicated method for obtaining desired resistivity, namely, altering the composition of the fully ordered alloy, is preferred. If, for example, a resistivity of 50 ohms/c.m.f. is desired, an alloy containing 20.5% copper and the balance essentially gold can be ordered to produce a wire having that resistivity. See

portion E-F of curve 11. Having reference to portion FG of curve '11, a completely ordered alloy containing 30% copper and the balance essentially all gold also has a resistivity of 50 ohms/emf. Ordered coppergold alloys of other specific compositions produce re sistance elements having the same resistivity as indicated on portions G-H and HI, of curve II.

The ordered alloys represented by portion EF of curve II have two important advantages. First, these alloys afford the greatest resistance to corrosion since they contain the highest percentage range of gold. Secondly, these alloys cover the widest range of resistivities obtainable in the series.

In instances where cost is an important consideration, ordered compositions represented by portion H-J of curve 11 are recommended. While such alloys are not as highly corrosion resistant, the are less expensive for the reason that they contain considerably smaller percentages of gold.

The generally accepted minimum life of low overall resistance potentiometers is one million cycles. Test potentiometers embodying windings of ordered copper-gold wires in accordance with the invention have operated satisfactorily after over 13.5 million cycles. The noise level is extremely low even with a very light wiper c011- tact pressure of 5 grams. The potentiometer resistance element of this invention is far superior to those of copper alloyed with a small percentage of nickel, which are the only other known available materials having requisite resistivity. By comparison, copper-nickel alloys are soft and subject to atmospheric corrosion causing noise and have poor wear characteristics as a potentiometer resistance element.

While specific embodiments of the invention have been shown and described in detail to illustrate the application of the invention principles, it will be understood that the invention may be embodied otherwise without departing from such principles.

I claim:

1. In a potentiometer or the like, a support, a resistance Wire wound on the support in a manner that successive convolutions thereof are closely adjacent each other, said resistance wire being composed of a binary alloy comprising from 15% to 56% copper by weight and the balance essentially gold, said resistance wire being ordered by heating the same for at least one half hour at an elevated temperature of about 700 F., followed by slow cooling, to reduce its resistivity, and a Wiper in contact with the resistance wire and movable with respect thereto.

2. A potentiometer or the like according to claim 1 wherein the resistance wire is composed of a binary alloy comprising from 15% to 25% copper by Weight and the balance essentially gold.

3. A potentiometer or the like according to claim 1 wherein the resistance wire is composed of a binary alloy.

References Cited in the file of this patent UNITED STATES PATENTS Lodge Jan. 8, 1935 Wilentchik July 20, 1954 OTHER REFERENCES Physical Chemistry of Metals, Darken and Gurry, McGraw-Hiil Book Company, Inc, 1953, p. 94. 

